tylernt:
[quote author=Coding Badly link=topic=128048.msg963356#msg963356 date=1350681198]If conserving energy is important, lower frequencies are better.
... unless you're spending most of your time sleeping. I suspect that running at a higher clock speed can allow you to complete your operations more quickly and get back to sleeping sooner, which may result in a net savings. I admit I have no hard data though.[/quote]
The theory is that there is a leakage loss when the processor is awake. Less time awake = less loss. The folks at avrfreaks.net have discussed it.
And while we're on the topic of clock speeds and fuses, I have a question about the three SUT (Start Up Time) options:
Don't we all. 
6CK + 0ms, 6CK + 4.1ms, and 6CK + 65ms. What does "6 CK" mean
Six clock cycles. I suspect it's a series of instructions the processor executes to wake-up.
and is it safe to use a +4.1ms setting with an 8MHz internal RC Oscillator?
"Safe" is a function of the power supply. The delay is meant to give the power supply time to stabilize. It's used in lieu of the BOD. If you know the supply will be stable when the processor wakes, short delays are safe. Or, if you use the BOD than the delay is not necessary. (At least that's my understanding from the datasheet description.)